δ15N of lipids in marine animals reflect the δ15N of the base of the food web
Stable isotopes of carbon and nitrogen (δ13C and δ15N) are now routinely used in trophic ecology. In particular, nitrogen isotopes are used to infer trophic transfers as each trophic step results in an increase in the δ15N. Provided that the δ15N of the base of the food web is known, trophic levels can then be inferred from this signal. Determining the δ15N at the base of the food web can, however, be difficult as e.g. it is often hard to measure the δ15N of primary producers. As an alternative, the δ15N of specific amino acids has been used to infer the δ15N of the base of the food web and subsequently to calculate trophic levels (e.g. Chikaraishi et al., 2007). Application of this technique is, however, limited as compound specific nitrogen stable isotope analysis is labor intensive and analytically relatively complex. In this study we investigated the δ15N of total lipid extract (TLE) of tissues of several marine animals as a tracer for the food web baseline.
The TLE was highly depleted in 15N compared to bulk biomass, and also highly variable (-14 to +0.7 ‰). Analysis by HPLC-ESI-MS showed that the TLE consisted mainly of phosphatidylcholines, a group of lipids with one nitrogen atom in the headgroup. To elucidate the cause for the 15N-depletion in the TLE, the δ15N of serine was determined as it is one of the main sources of nitrogen to N-containing lipids. Serine δ15N values correlated well with the 15N depletion in TLEs (R2=0.95). Previous studies have shown that serine is a so-called source amino acid, meaning that the δ15N of serine change relatively little during trophic transfer). The variability we observe for the 15N depletion of the TLE vs. bulk would thus primarily be caused by changes in the δ15N of bulk biomass during trophic transfer, while the δ15N of the lipid extract remains relatively unaffected. Indeed, we observe a strong correlation between the depletion in 15N of the lipid extract relative to biomass and trophic level (R2=0.69), suggesting that the analysis of δ15N of lipid extracts could potentially, in combination with the δ15N of the bulk biomass, provide a rapid tool to estimate trophic levels.
Our results suggest a new approach for trophic ecology involving measurement of the δ13C of lipid-extracted biomass, the δ15N of total lipid extract and the δ15N of bulk biomass.